Boron-doped diamond coating and diamond-coated tool

ABSTRACT

A boron-doped diamond coating that is to be disposed on a surface of a body includes a plurality of diamond microcrystal whose crystal grain diameter is substantially not larger than 2 μm, wherein the diamond microcrystal is doped with boron.

This application is based on Japanese Patent Application No. 2004-349150 filed Dec. 1, 2004, the contents of which are incorporated hereinto by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a boron-doped diamond coating that is to be disposed on a surface of a body of such as a tool, especially, to an improvement in oxidation resistance and lubricity.

2. Description of Related Art

A diamond-coated tool coated with a diamond coating on the body made of such as cemented carbide is suggested for a cutting tool such as an end mill, a turning tool, a tap, a drill or other tools. JP 2519037B and JP 2002-79406A disclose examples of such tools. These diamond-coated tools have very high hardness, superior wear resistance and adhesion resistance. JP 2004-193522A and JP 10-146703A disclose technology of doping with boron (B) upon the crystal growth of the diamond by such as the microwave plasma CVD (chemical vapor deposition) method in order to let it have electrical conductivity and to improve in oxidation resistance.

Doping with boron for the diamond coating disposed on the surface of such as a tool body, however, has not been suggested yet. Inferior oxidation resistance and lubricity frequently cause the insufficiency of the diamond coating in durability by oxidation and early wearing in cutting of composite material including iron series material and heat-resistant alloy such as titanium alloy suffering at high temperature on its cutting point. And heat by friction frequently causes low durability of the diamond coating and damages in quality on the cut surface of the workpiece.

It is therefore an object of the present invention to provide a boron-doped diamond coating that is to be disposed on a surface of a body of such as a tool, especially, an improvement in oxidation resistance and lubricity.

SUMMARY OF THE INVENTION

The object indicated above may be achieved according to a first aspect of the invention, which provides a boron-doped diamond coating that is to be disposed on a surface of a body comprising a plurality of diamond microcrystals whose crystal grain diameter is substantially not larger than 2 μm, wherein the diamond microcrystal is doped with boron.

The object indicated above may be achieved according to a second aspect of the invention, which provides the boron-doped diamond coating according to the first aspect of the invention, wherein each crystal grain diameter of at least not less than 80% of the number of the diamond microcrystals is not larger than 2 μm.

The object indicated above may be achieved according to a third aspect of the invention, which provides the boron-doped diamond coating according to the first or second aspect of the invention, wherein the boron-doped diamond coating is doped with boron in a ratio of 0.05-2.0 atomic % of boron.

The object indicated above may be achieved according to a fourth aspect of the invention, which provides the boron-doped diamond coating according to third aspect of the invention, wherein a layer of boron oxide is formed on a surface of the boron-doped diamond coating when the surface is oxidized.

The object indicated above may be achieved according to a fifth aspect of the invention, which provides the boron-doped diamond coating according to any one of the first through fourth aspects of the invention, wherein a maximum height Rz of the boron-doped diamond coating is not larger than 0.7 μm.

The object indicated above may be achieved according to a sixth aspect of the invention, which provides the boron-doped diamond coating according to any one of the first through fifth aspects of the invention, wherein a thickness of the boron-doped diamond coating is in a range of 5-25 μm.

The object indicated above may be achieved according to a seventh aspect of the invention, which provides the boron-doped diamond coating according to any one of the first through sixth aspects of the invention, wherein the body is made of cemented carbide. The object indicated above may be achieved according to an eighth aspect of the invention, which provides the boron-doped diamond coating according to any one of the first through sixth aspects of the invention, wherein the body is made of high-speed steel.

The object indicated above may be achieved according to a ninth aspect of the invention, which provides a diamond-coated tool comprising a main body having a surface, and the boron-doped diamond coating according to any one of the first through eighth aspects of the invention, wherein the boron-doped diamond coating is disposed on the surface of the main body.

The object indicated above may be achieved according to a tenth aspect of the invention, which provides the diamond-coated tool according to the ninth aspect of the invention, wherein the diamond-coated tool is an end mill. The object indicated above may be achieved according to an eleventh aspect of the invention, which provides the diamond-coated tool according to the ninth aspect of the invention, wherein the diamond-coated tool is a turning tool. The object indicated above may be achieved according to a twelfth aspect of the invention, which provides the diamond-coated tool according to the ninth aspect of the invention, wherein the diamond-coated tool is a tap. The object indicated above may be achieved according to a thirteenth aspect of the invention, which provides diamond-coated tool according to the ninth aspect of the invention, wherein the diamond-coated tool is a drill.

The boron-doped diamond is a diamond in which one or a plurality of carbon atoms are replaced by one or a plurality of boron atoms and serves as a p-type semiconductor having a positive hole with positive charge. “Atomic % (percent)” is defined as the number of atoms of a particular element present in every hundred atoms within the detection volume. The “atomic %” of boron means a ratio of the number of atoms replaced by boron atoms to the number of the total of the boron atoms and other atoms (that is, all atoms) and it is measured by such as the secondary ion mass spectrometry.

In this boron-doped diamond coating, a layer of boron oxide such as B₂O₃ is formed on the surface when the surface is oxidized, and therefore the progression of oxidation is blocked by the layer of boron oxide. This provides a high oxidation resistant and lubricative coating with a lower coefficient of friction. Especially further superior lubricity is achieved in this invention with a further lower coefficient of friction because the surface of the diamond coating including diamond microcrystals in this invention is more even than that of the conventional diamond coating and furthermore the layer of boron oxide is formed on that surface. This provides superior durability by preventing flaking or early wearing of the diamond coating by oxidation in cutting of the composite material including a material of iron series or in cutting of the heat-resistant alloy such as a titanium alloy with the cutting point at a high temperature. And high lubricity in this invention preventing heat generation by friction contributes to high durability of the diamond coating and advancement in quality of the worked surface of the workpiece.

The diamond-coated tool that the above boron-doped diamond coating is disposed on the surface of the body according to the third aspect of the invention substantially achieves the effects as described above.

While the boron-doped diamond coating according to the invention is preferably applied to a tool such as a cutting tool requiring wear resistance, oxidation resistance and lubricity, namely, a diamond coating tool, it may be also applied to other use than tools, for example, to a hard coating for such as a semiconductor apparatus.

While hard tool material such as cemented carbide is preferably used for the body to be coated with boron-doped diamond coating for the diamond-coated tool, other tool materials such as high-speed steel may be used. The predetermined pretreatment as the surface roughening treatment or coating of other coating as a substrate on the surface of the tool body may be conducted for high adhesion.

The thickness of the boron-doped diamond coating is preferably determined 5-25 μm, appropriately 10-20 μm, because the thinner coating than 5 μm does not have sufficient wear resistance and the thicker coating than 25 μm is apt to be flaked. The thickness for other use than tools may be appropriately determined according to the kind of the coated material, the object and others. This invention may include the laminated coating with the boron-doped diamond coating and the hard coating made of intermetallic compound such as TiAlN or other coating one after the other upon the condition that the boron-doped diamond coating is coated at the top of the coatings.

The CVD (chemical vapor deposition) is preferably used for coating of the boron-doped diamond coating, especially the microwave plasma CVD is preferable, while other CVDs such as the hot filament CVD or radio-frequency plasma CVD may be used. Conventional various methods for doping diamond with boron, for example, disclosed in JP 2004-193522A and JP 10-146703A, may be adopted.

The boron-doped diamond microcrystal coating can be formed by repeating of nucleation steps and crystal growth steps as disclosed in JP 2002-79406A. The crystal grain diameter is preferably determined to be not larger than 2 μm, more preferably not larger than 1 μm. This crystal grain diameter means the maximum diameter as measured in a perpendicular direction to the crystal growth direction. While the crystal grain diameters of not larger than 2 μm of all diamond crystals are preferable, the crystal grain diameters of not larger than 2 μm of at least not less than 80% of (the number of) the diamond crystals (or diamond microcrystals or the microcrystalline diamonds) in a plane of the surface or a predetermined sectional plane are acceptable or sufficient. In general the crystal grain diameter of the diamond crystal is not larger than 2 μm in the perpendicular direction to the crystal growth direction if the length of the diamond crystal in the crystal growth direction is controlled to be not larger than 2 μm. The crystal grain diameters of not larger than 2 μm of the diamond crystals are acceptable or sufficient if the length of the diamond crystal in the crystal growth direction exceeds 2 μm.

0.05-2.0 atomic % of boron doping (content) is appropriate because less than 0.05 atomic % of boron doping does not achieve sufficient effects of oxidation resistance and lubricity and more than 2.0 atomic % of boron doping impairs the original characteristics of the diamond coating with respect to such as wear resistance, and 0.5-1.0 atomic % of boron doping is preferable, while more than 2.0 atomic % of boron doping may be adopted for achieving the above effects. Various embodiments with respect to the amount of boron for doping may be adopted. It is not necessary that the boron-doped diamond coating has the constant amount for doping in every portion within the coating. For example, an embodiment having a continuous or staged increase of doping as approaching the surface of the coating or an embodiment having a multilayer structure mutually laminated with the layer doped with the large amount of boron and the layer doped with the small amount of boron, or other embodiment may be acceptable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an end mill in an embodiment according to the invention in a front view as viewed from the perpendicular direction to the axis of the end mill;

FIG. 1B illustrates an adjacent portion to the surface of a cutting edge of the end mill in FIG. 1A in a sectional view;

FIG. 2 illustrates an example of the microwave plasma CVD apparatus used for forming the diamond coating in a diagram;

FIG. 3 illustrates a flowchart showing steps for forming the diamond microcrystal coating by the apparatus in FIG. 2;

FIG. 4A illustrates a diagram based on an electron micrograph showing a surface of a diamond coating made of (coarse) crystals having a conventional crystal grain diameter;

FIG. 4B illustrates a diagram based on an electron micrograph showing a surface of a diamond microcrystal coating according to the invention;

FIG. 5A illustrates a graph of the contour curve showing coarseness of a surface of a diamond coating made of (coarse) crystals having a conventional crystal grain diameter;

FIG. 5B illustrates a graph of the contour curve showing coarseness of a surface of a diamond microcrystal coating according to the invention;

FIG. 6A illustrates the condition for the test in durability between the boron-doped and non-boron-doped diamond coatings;

FIG. 6B illustrates the result of the test in durability between the boron-doped and non-boron-doped diamond coatings;

FIG. 7A illustrates the condition for the test for the coefficient of friction between the boron-doped and non-boron-doped diamond coatings;

FIG. 7B illustrates the result of the test for the coefficient of friction between the boron-doped and non-boron-doped diamond coatings;

FIG. 8 illustrates the result of the test for the ratio of mass change showing a degree of oxidation between the boron-doped and non-boron-doped diamond coatings in the atmospheres of various temperatures;

FIG. 9 illustrates a diagram based on a photograph showing an external appearance of a square end mill having two edges coated with the boron-doped diamond coating;

FIG. 10 illustrates a diagram based on a photograph showing external appearances of two square end mills respectively having the boron-doped (right) and non-boron-doped (left) diamond coatings after the test of oxidation;

FIG. 11 illustrates a turning tool in an embodiment according to the invention;

FIG. 12 illustrates a tap in an embodiment according to the invention;

FIG. 13 illustrates a drill in an embodiment according to the invention;

FIG. 14 illustrates a form rolling tool (plastically deforming tool) in an embodiment according to the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, there will be described an abrasive body by reference to the drawings. FIG. 1A illustrates an end mill 10 according to the invention in a front view as viewed from the perpendicular direction to the axis of the end mill 10 and FIG. 1B illustrates an adjacent portion to the surface of a cutting edge 14 of the end mill 10 in FIG. 1A in a sectional view. This end mill 10 is a square end mill having four cutting edges. A tool substrate (or a body) 12 of the end mill 10 is made of cemented carbide and the tool substrate 12 has a shank and the cutting part 14 that are integrally formed. The cutting part 14 corresponds to the main body and is provided with peripheral cutting edges 16 and end cutting edges 18 as cutting edges. The cutting part 14 is coated with the boron-doped diamond coating (hereinafter in some cases referred to as diamond coating) 20 that has a multilayer structure constituted of microcrystals of their crystal grain diameter of not larger than 1 μm and is doped with boron in a ratio of 0.5-1.0 atomic % of boron to all atoms in the boron-doped diamond coating. The thickness of the diamond coating 20 is approximately 20 μm. The oblique lines in FIG. 1A shows the area corresponds to the surface of the tool substrate 12 coated with the diamond coating 20.

The tool substrate 12 having the peripheral cutting edges 16 and end cutting edges 18 is formed by such as grinding of a material of cemented carbide, then the surface of the cutting part 14 of the tool substrate 12 is treated with the surface roughening treatment and this causes high adhesion of the diamond coating 20 to complete the end mill 10. For the surface roughening treatment, for example, a method utilizing chemical corroding such as electropolishing or sand blasting with such as abrasive grains of SiC or the like is appropriately adopted. Then the roughened surface of the cutting part 14 is coated with the diamond coating 20 by the vapor phase synthesis method, for example, by the microwave plasma CVD method using the microwave plasma CVD apparatus in FIG. 2 to generate and grow the diamond particles or crystals with doping with boron.

The microwave plasma CVD apparatus 30 shown in FIG. 2 is provided with a reactor 32, a microwave generator 34, a source gas supply device 36, a vacuum pump 38 and an electromagnetic coil 40. In the cylindrical reactor 32 is provided a table 42, a workpiece support 44 on the table 42 supports a plurality of tool substrates 12 to be coated with the diamond coating 20 and each of the tool substrate 12 is arranged so that the cutting part 14 of the tool substrate 12 is at the upper side. The microwave generator 34 generates such as 2.45 GHz microwave and the introduced microwave into the reactor 32 causes the tool substrate 12 to be heated. Controlling of the power of the microwave generator 34 allows the regulation of the temperature for heating.

The source gas supply device 36 supplies source gases such as methane (CH₄), hydrogen (H) and carbon monoxide (CO) to the reactor 32 and the apparatus 36 is provided with such as a gas container for each of the source gas, a flow control valve for controlling the flow of the source gas and a flowmeter. In this embodiment the apparatus 36 is capable of supplying the source gas mixed with such as a liquid of methanol in which boron oxide is dissolved for doping with boron. The vacuum pump 38 is provided for depressurizing by sucking a gas in the reactor 32. Feedback control of such as the motor current of the vacuum pump 38 allows the value of the pressure in the reactor 32 measured by the pressure gauge 46 to be maintained at the predetermined value of the pressure. The electromagnetic coil 40 is provided annularly in the outer circumference of the reactor 32 as surrounding the reactor 32.

The coating treating of the diamond coating 20 by the microwave plasma CVD apparatus 30 includes the nucleus adhesion step R1 and the crystal growth step R2 in FIG. 3. In step R1 of the nucleus adhesion step, after the regulation of the methane and hydrogen flows for the predetermined value in the range of 10-30% of methane in concentration, the regulation of the operation of the microwave generator 34 for the predetermined temperature on the surface of the tool substrate 12 in the range of 700-900° C., and the regulation of the operation of the vacuum pump 38 for the predetermined pressure in the reactor 32 in the range of 2.7×10²−2.7×10³ Pa, the condition is maintained with the predetermined value of methane in concentration, at the predetermined temperature and under the predetermined pressure for 0.1-2 hours. This step R1 provides the deposition of the nucleus layer as the start point of the diamond crystal growth on the surface of the tool substrate 12 or on the surfaces of a plurality of diamond crystals that the crystal has grown in the crystal growth treatment in step R2.

In step R2 of the crystal growth step, after the regulation of the methane and hydrogen flows for the predetermined value in the range of 1-4% of methane in concentration, the regulation of the operation of the microwave generator 34 for the predetermined temperature on the surface of the tool substrate 12 in the range of 800-900° C., and the regulation of the operation of the vacuum pump 38 for the predetermined pressure in the reactor 32 in the range of 1.3×10³−6.7×10³ Pa, the condition is maintained with the predetermined value of methane in concentration, at the predetermined temperature and under the predetermined pressure for the predetermined duration of time. That predetermined duration of time is determined such that the crystal grain diameter of the diamond crystal is maintained not larger than 1 μm in diameter, in other words, it is shorter than another predetermined duration of time that the diamond crystal grows to 1 μm in length, namely, in the length dimension in the direction of crystal growth. In the crystal growth treatment of this embodiment the crystal grain diameter of the diamond crystal in the plane substantially perpendicular to the crystal growth direction is maintained not larger than 1 μm if the length dimension in the crystal growth direction is not larger than 1 μm.

In the next step R3 it is judged by such as the repeated times of step R2 whether the thickness of the diamond coating 20 formed on the surface of the tool substrate 12 with the crystal growth treatment reaches the predetermined value, for example, 20 μm in this embodiment, and the treatments in steps R1 and R2 is repeated until the thickness reaches the predetermined value. In the treatment of step R1 the diamond crystal growth stops and on the grown crystals a new layer of the nucleus is formed. Later in the crystal growth treatment, step R2, the diamond crystals under the layer of the nucleus do not grow, a new diamond crystal grows from the new nucleus as the start point, and therefore the tool substrate 12 is coated with the multilayer diamond coating 20 with microcrystals of not larger than 1 μm both in diameter and length, namely, of not larger than 1 μm crystal grain diameter and crystal length.

In the coating treatment the diamond coating 20 is doped with 0.5-1.0 atomic % of boron by supplying the source gas such as hydrogen mixed with the liquid of methanol dissolved with the boron oxide to the reactor 32 in the predetermined flow rate. The regulation of the doping amount of boron is achieved by controlling the flow rate of the supply of the liquid with boron oxide dissolved.

Since the diamond coating 20 disposed on the end mill 10 of this embodiment is doped with 0.5-1.0 atomic % of boron, a layer of boron oxide such as B₂O₃ is formed on the surface when the surface is oxidized, and therefore the progression of oxidation is blocked by the layer of boron oxide. This provides a high oxidation resistant and lubricative coating with a lower coefficient of friction. Especially further superior lubricity is achieved in this embodiment with a further lower coefficient of friction because the surface of the diamond coating 20 including diamond microcrystals of the crystal grain diameter of not larger than 1 μm in this embodiment is more even than that of the conventional diamond coating 20 and furthermore the layer of boron oxide is formed on that surface. This provides superior durability by preventing flaking or early wearing of the diamond coating 20 by oxidation in cutting of the composite material including a material of iron series or in cutting of the heat-resistant alloy such as a titanium alloy with the cutting point at a high temperature. And high lubricity in this embodiment preventing heat generation by friction contributes to high durability of the diamond coating 20 and advancement in quality of the worked surface of the workpiece.

FIG. 4A illustrates a diagram based on an electron micrograph showing a surface of a boron-doped diamond coating made of (coarse) crystals having a conventional crystal grain diameter, namely, the coating having the predetermined thickness as a result of the diamond crystal growth in one crystal growth treatment, step R2 in FIG. 3. FIG. 4B illustrates a diagram based on an electron micrograph showing a surface of a diamond coating 20 according to this embodiment. The difference in the crystal grain diameter of the diamond crystal is apparent from the diagrams of FIGS. 4A and 4B.

FIG. 5A illustrates a graph of the contour curve showing coarseness or evenness of a surface of the same boron-doped diamond coating made of (coarse) crystals having a conventional crystal grain diameter as shown in FIG. 4A. And FIG. 5B illustrates a graph of the contour curve showing coarseness of a surface of the diamond coating 20 according to this embodiment. The maximum height Rz of the coating in FIG. 5A is 3.0 μm and the maximum height Rz of the coating of the present invention in FIG. 5B is 0.7 μm. Thus the present invention provides very even surface of the coating and therefore advancement in the evenness of the worked surface of the workpiece is expected to a high degree.

FIG. 6A illustrates the condition for the test in durability between the drill coated with the same diamond coating as the diamond coating 20 according to the present invention and the conventional drill with the diamond microcrystal coating thereon without doping with boron (or the drill with non-boron-doped diamond coating) and FIG. 6B illustrates the result of the same test. This is the durability test in cutting aluminum alloy, ADC 12. This test reveals the boron-doped diamond coating according to the present invention in approximately twice the durability comparing to the conventional non-boron-doped diamond coating.

FIG. 7A illustrates the condition for the test for the coefficient of friction of pins coated with the non-boron-doped diamond coating made of (coarse) crystals having a conventional crystal grain diameter, the non-boron-doped diamond microcrystal coating and the boron-doped diamond microcrystal coating formed in the same condition as the diamond coating 20 and FIG. 7B illustrates the result of the test. This test reveals the boron-doped diamond microcrystal coating according to the present invention having a smaller coefficient of friction than the coarse crystal diamond coating and the non-boron-doped diamond microcrystal coating. This is regarded as the effect of the formed boron oxide layer on the diamond coating.

FIG. 8 illustrates the result of the test for the loss (%) of mass by oxidation, that is, the ratio of mass change denoting a degree of oxidation, between the 0.5-1.0 atomic % boron-doped and non-boron-doped diamond coating made of (coarse) crystals having a conventional crystal grain diameter in the atmospheres of various temperatures with removing only the coating from the substrate, measuring the mass of the coating, then heating it, and measuring the mass of the coating after heating. In the test the change of mass is measured by such steps as heating the coating to each of the predetermined temperature, such as 700, 725, 750, 775 and 800° C. respectively, by a 15° C./min. increase, maintaining it at the same predetermined temperature for 30 minutes and leaving it until the temperature of it lowers to the room temperature. As apparent in FIG. 8 oxidation begins at approximately 700° C. in the non-boron-doped diamond coating and oxidation begins at approximately 775° C. in the boron-doped diamond coating with a difference of approximately 75° C. between them. While the diamond coating made of (coarse) crystals having a conventional crystal grain diameter was used in this test, the substantially same result is expected in the case using the diamond microcrystal coating according to the present invention because oxidation resistance is expected to depend upon the presence of boron which the diamond coating is coated with.

FIG. 9 illustrates a diagram based on a photograph showing an external appearance of a square end mill having two edges coated with the 20 μm thick boron-doped diamond coating made of (coarse) crystals having a conventional crystal grain diameter which is doped with 0.5-1.0 atomic % of boron. And FIG. 10 illustrates a diagram based on a photograph showing external appearances of two square end mills respectively having the boron-doped diamond coating (right) in FIG. 9 and the 20 μm thick non-boron-doped diamond coating (left) made of (coarse) crystals having a conventional crystal grain diameter after the test of oxidation. In the oxidation test the condition of the coating is inspected, that is, the loss or the lost area of the coating is measured by such steps as heating the coating to 750° C. by a 15° C./min. increase, maintaining it at 750° C. for 30 minutes and leaving it until the temperature of it lowers to the room temperature. While the left end mill of the non-boron-doped diamond coating has lost approximately 100% of the diamond coating that was removed or flaked caused by the difference of thermal expansivity between the coating and the tool substrate or oxidation, the right end mill of the boron-doped diamond coating has lost only approximately 10% of the diamond coating that was removed or flaked and almost the diamond coating remains in FIG. 10. The black area in FIG. 10 denotes the diamond coating and 17-18 μm thick diamond coating remained on the bottom edge at the end of the right end mill of the boron-doped diamond coating. While the diamond coating made of (coarse) crystals having a conventional crystal grain diameter was used in this test, also in this test the substantially same result is expected in the case using the diamond microcrystal coating according to the present invention because oxidation resistance is expected to depend upon the presence of boron which the diamond coating is coated with.

FIG. 8 shows the loss of 0% of the coating for the end mill of the boron-doped diamond coating and the loss of 8-10% of the coating for the end mill of the non-boron-doped diamond coating at 750° C. Excluding the effect caused by the difference of thermal expansivity between the coating and the tool substrate is expected to introduce the less loss of the coating than that in the test in FIG. 10.

FIG. 11 illustrates a turning tool 100 in an embodiment according to the invention. FIG. 12 illustrates a tap 102 in an embodiment according to the invention. FIG. 13 illustrates a drill 104 in an embodiment according to the invention. FIG. 14 illustrates a form rolling tool (or plastically deforming tool) 106 in an embodiment according to the invention. The oblique lines in these FIGS. 11-14 show the areas corresponding to the surface coated with the hard coating 20.

It is to be understood that the present invention may be embodied with other changes, improvements, and modifications that may occur to a person skilled in the art without departing from the scope and spirit of the invention defined in the appended claims. 

1. A boron-doped diamond coating that is to be disposed on a surface of a body comprising: a plurality of diamond microcrystal whose crystal grain diameter is substantially not larger than 2 μm, wherein the diamond microcrystal is doped with boron.
 2. The boron-doped diamond coating according to claim 1, wherein each crystal grain diameter of at least not less than 80% of the number of the diamond microcrystals is not larger than 2 μm.
 3. The boron-doped diamond coating according to claim 1, wherein the boron-doped diamond coating is doped with boron in a ratio of 0.05-2.0 atomic % of boron.
 4. The boron-doped diamond coating according to claim 3, wherein a layer of boron oxide is formed on a surface of the boron-doped diamond coating when the surface is oxidized.
 5. The boron-doped diamond coating according to claim 1, wherein a maximum height Rz of the boron-doped diamond coating is not larger than 0.7 μm.
 6. The boron-doped diamond coating according to claim 1, wherein a thickness of the boron-doped diamond coating is in a range of 5-25 μm.
 7. The boron-doped diamond coating according to claim 1, wherein the body is made of cemented carbide.
 8. The boron-doped diamond coating according to claim 1, wherein the body is made of high-speed steel.
 9. A diamond-coated tool comprising: a main body having a surface; and the boron-doped diamond coating defined in claim 1, wherein the boron-doped diamond coating is disposed on the surface of the main body.
 10. The diamond-coated tool according to claim 9, wherein the diamond-coated tool is an end mill.
 11. The diamond-coated tool according to claim 9, wherein the diamond-coated tool is a turning tool.
 12. The diamond-coated tool according to claim 9, wherein the diamond-coated tool is a tap.
 13. The diamond-coated tool according to claim 9, wherein the diamond-coated tool is a drill. 